A study of lithium ion batteries cycle aging by thermodynamics techniques

It is found that with increasing cycle number, the entropy and enthalpy profiles show more significant changes than those observed in the discharge and the OCP curves especially at particular SOC and OCP values. These differences are attributed to the higher sensitivity of entropy and enthalpy state functions to changes in the crystal structure of the graphite anode and the lithiated cobalt oxide (LCO) induced by cycle aging compared to the free energy ΔG (OCP) alone. The thermodynamics data are supported by post-mortem X-ray diffractometry (XRD) and Raman scattering (RS) analyses on the electrode materials. The results show important LCO crystal structure degradation, whereas, surprisingly, the graphite anode remains almost unaffected by heavy cycling, if not improved.